I was thinking to create a generic base class and have each of the module base class inherit from it... but is there a better way to do this?
I'd say if you need a base class, you're not having real modules. I think modules should be "totally" independent sub-applications.
If you really want to write such a base class, use dependency injection, not inheritance so you can inject the needed common behaviour in all modules without creating such a tight coupling of all your modules. Just create a subfolder under 'application' and call it 'Base' or something. But again, I think it might not be such a good idea resp. if you need it, we're not talking about modules. Then again, each case is different.
Related
In SystemVerilog, can parent/child inheritance be accomplished for modules?
For instance if I have data for a protocol header, I can create a class header, and variations that inherit from it, such as long_header and short_header. The parent class can have a method like set_parity, and variations can optionally be defined for long or short headers as needed, like long_header.set_parity
Modules to process these headers, like "long_header_processor" would then use the long_header.set_parity method.
Is this how it's done?
Is there an alternative way to obtain this inheritance, to somehow have a "long_header_processor" module inherit from a parent "header_processor" module?
No, in SystemVerilog, modules are a much more 'hardwary' construct, there's no concept of inheritance for modules. They can be parameterizable, but that's it. Classes and modules are very different concepts in SV - modules are typically used in design, and classes in verification.
I've noticed that all of the examples in Optaplanner use the line
import org.optaplanner.examples.common.domain.AbstractPersistable;
to import an abstract class that most of the domain classes implement. To persist my solution to an xml file, should I also utilize this class? My maven project would need to have org.optaplanner.examples as a dependency, which I am hesitant to do.
I've read the source for AbstractPersistable, and I can't tell what benefits it provides to the example projects.
Don't depend on optaplanner-examples.
You probably don't need that class, just leave it out.
If you want to have that superclass that provides a database id, for every domain class, simply copy paste that class into your sources.
In java, there is a way to import a class and all of its children in one line:
import java.utils.*
In Perl, I've found I can only import specific classes:
use Perl::Utils::Folder;
use Perl::Utils::Classes qw(new run_class);
Is there similar way like java to import everything that falls under a tree structure, only in Perl?
No, there is not a way to easily do what you are after.
You could walk the relevant paths in your PERL library's filesystem and use every .pm file you came across (that's what Module::Find, as suggested by #Daniel Böhmer, does), but that can miss a few things:
Packages that are declared in funny ways/at runtime.
Multiple packages per module file.
Other cases I haven't thought of.
This is also a bad idea, for a few reasons:
You mentioned "classes" in your question, rather than just packages. Perl packages and subpackages do not necessarily represent classes/instantiable object-oriented code. If you were to programmatically generate a list of all packages in a hierarchy and then call $packagename->new() on each of them, you might have a syntax error, if one of the packages was just a library of functions.
Packages and subpackages often are not directly related, developed by the same people, or used for similar things. Just because a package starts with Net:: doesn't mean that it will obey standard conventions that other Net::-prefixed packages expect. For example, File::Find and File::Tail share a prefix, but have very little to do with each other; the prefix is in common because both utilities work with files as their goal.
Lots of packages do things at BEGIN/INIT/etc time when they're compiled. Some of them (sadly) do different things depending on the order in which they're used relative to other modules. The solution to this problem for module developers is "don't do that", but for module users, it's "use sparingly, and only when needed".
It clutters your local namespace with lots of potentially-exported symbols you don't necessarily need (to conditionally import symbols, you'll have to use import arguments like you're doing in your example; there's no programmatic way to define "symbols I'm interested in", since Perl doesn't have that kind static analysis at compile time . . . not for lots of call styles, at least).
It slows down your program's startup time by compiling things you might not necessarily need. This might not seem important at the early phase of a project, but for larger projects it is very easy to end up in situations where you're pulling in over a thousand CPAN modules when you start Apache (or launch your main script, or whatever), and your app takes more than a minute just to start.
I have a hunch that you're trying to reduce boilerplate (as in: all of your modules have a big block of use statements at the top, and that's duplicated everywhere). There are a few ways to do this, starting with:
Don't: import things in each module as you need them, and use strict/warnings and lots of tests to be told early on if you're calling functionality that you haven't imported yet.
You could also make your own Exporter subclass that uses all of your standard modules and adds the functions that you frequently use from them to its #EXPORTS (or splices their #EXPORTS onto its own, or uses Exporter sub-sub-classing, or something).
Factor your code so that the parts that depend on multiple imported modules live in a single utility module, and import that.
Factor your code so that the parts that depend on the imported modules live in a parent class, and address its methods via instances of subclasses (or SUPER), so your subclasses don't have to explicitly contain the imports, e.g. $instance->method_that_calls_an_imported_function_in_the_parent();
Also, as an aside, using package.* imports in Java is debatable, and has many of the same drawbacks of doing it in Perl.
In Perl, the class Foo::Bar::Foo may not be a subclass of Foo::Bar. Nor, is there any guarantee that a subclass module even has the same class prefix. IO::File is a subclass of IO::Handle and not of IO which isn't even a module.
There also isn't even an easy way to tell of a Perl module is a sub-class of another Perl module. There are (at least) three ways to declare a subclass' relationship to a class:
use parent
use base
The #ISA package variable
It is possible to use #INC to find all modules, then look at the source and look at use parent, use base, and #ISA declarations and build a Perl class matrix, then go through that matrix to load the classes you do need. That will probably be slow and cumbersome, and doesn't even cover Moose based classes.
You're asking the wrong question. You're asking "Find all of the subclasses of a particular class.". This will include classes that you're probably not even interested in. I know (for example LWP) that there can be dozens of various classes and subclasses that include stuff you're not even interested in.
What you should be asking is "What do I need to do?", and then find the classes that fulfill your needs. If these classes happen to be child classes of a particular parent class, these subclasses will load the required class.
We do Java programming here, and one of the standards is not to use asterisks in our import statements. This is considered sloppy programming. If you need a particular class, you should declare it rather than simply declaring a superclass. Many of our reporting tools have problems with asterisk declarations in import statements.
There is a Module::Find module, but I am not sure exactly how it works. I believe it simply assumes that subclasses are in the same module hierarchy as the superclass, but that's far from true in Perl.
In general, I think it is a bad idea to load a whole 'tree' of modules (or subclasses so to speak).
There is definitely something wrong in your design if you need to know all and everything about sub classes/modules. You break the rules of encapsulation and you should not need to know how a class handles its responsibilities.
To me classes are quite similar to NodeJS (CommonJS) modules. You can have many of them, they can be reused, they can use each other and they are generally one-per-file.
What makes modules so different from classes? The way you use them differs, and the namespace difference is obvious. Besides that they seem very much the same thing to me or perhaps I am just not seeing the obvious benefit here.
Modules are more like packages (to use the Java term) than classes. You don't instantiate a module; there is only one copy of it. It's a tool for organizing related functionality, but it doesn't typically encapsulate the data of a particular instance of an object.
Probably the closest analogue to a class (setting aside those libraries that actually construct class-based inheritance in JavaScript) is just a constructor function. You can of course put such functions inside a module.
function Car() {
this.colour = 'red';
}
Car.prototype.getColour = function() { return this.colour; };
var myCar = new Car();
myCar.getColour(); // returns 'red'
You use both modules and classes for encapsulation, but the nature of that encapsulation is different.
JS was initially a prototypal inheritance system. It was super simple like the rest of the language. But then Netscape decided to make it be more like Java and added the idea of constructors to the language. Hence pseudo classes were born.
You can check this link to know how prototypal OOP is used in JS:
http://howtonode.org/prototypical-inheritance
One critical thing; that "generally one-per-file" thing is not true; modules are absolutely one-per-file. A require() that brings the module's exports into the namespace has no way of distinguishing between the exported contents of that module; everything that module (file) exports are imported with a require() statement. Attempting to put more than one module into a file only means that you'll get everything in that file when you try to load "either" module.
I've recently started to try to use Dist::Zilla for maintaining Path::Class. I added the [PodCoverageTests] plugin, and it's reporting some failures in the Path::Class::Entity class, which is the abstract base class for Path::Class::File and Path::Class::Dir.
What I'd like is some way to tell the testing code that Entity doesn't need docs, but its two derived classes do - even though the methods are only defined in the parent class. Anyone know some way to do that?
Dist::Zilla's standard POD coverage test uses Pod::Coverage::TrustPod.
You should be able to say
=for Pod::Coverage
.
to tell it to assume that everything is documented.